Apparatus and Method for Measuring Relative Frequency Response of Audio Device Microphones

ABSTRACT

Test apparatus measuring relative frequency response of first and second microphones includes a rotatable carrier. First and second microphones are sealingly clamped against a mounting surface of the carrier aligned with first and second apertures therein, such apertures lying equidistant from, and on opposite sides of, the carrier&#39;s axis of rotation. The carrier initially positions the first microphone closest to an audible signal source, and the responses of the microphones to an audible excitation signal are measured. The carrier is rotated 180 degrees, and the measurements are repeated. Elongated strips of gasket material are used to align the microphones and to form a seal with the carrier. When microphones are mounted deep within an audio device, the audio device is sealingly clamped against a mounting plate, sequentially aligning the mounting plate aperture with first and second apertures of the audio device housing corresponding to first and second microphones disposed therein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending parent U.S. patentapplication Ser. No. 14/454,576, filed on Aug. 7, 2014, for “Apparatusand Method for Measuring Relative Frequency Response of Audio DeviceMicrophones”, and the benefit of the earlier filing date of such parentU.S. patent application Ser. No. 14/454,576 is claimed hereby under 35U.S.C. §120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to audio devices whichincorporate microphones for sensing sound, and more particularly, to anapparatus and method for measuring the relative frequency response oftwo or more microphones provided within such an audio device tofacilitate a fixed calibration compensation system.

2. Description of the Relevant Art

A microphone is one of the key components in many audio products,including those used for telecommunications. A microphone is atransducer that converts acoustic (sound) energy into electrical energy.It is known to employ speech enhancement algorithms and/or noisereduction algorithms within such products to process incoming signalsfrom a microphone to enhance the performance of such products inacoustically challenging environments, e.g., in the presence of unwantedbackground noise.

Until recently, the majority of consumer electronics used only a singlemicrophone. With rapid advances in high speed digital signal processors,speech enhancement algorithms and noise reduction algorithms havestarted using two or more microphones to exploit the spatial diversitythat exists between such microphones. In certain scenarios, thesemultiple-microphone-based algorithms can provide sound quality farsuperior to single microphone implementations. Today, speakerphones,conference phones used to conduct telephone conference calls in anoffice conference room, and even Bluetooth® telephone earpieces, oftenemploy two or more microphones to sense surrounding sounds. In the caseof conference phones, the use of multiple microphones, together withdigital signal processing, helps to ensure that all speakers aredetected while allowing the audio device to focus on the active speakerat any given point in time. The use of multiple microphones is also keyto achieving echo cancellation and suppression of unwanted backgroundnoise signals.

However, the improved performance of these multiple-microphone-basedalgorithms introduces many new problems. For example, it has been foundthat audio devices that use multiple microphones to achieve speechenhancement and/or noise reduction perform poorly if the frequencyresponses of such microphones are not well matched. If the microphonesused within a particular audio device are well matched to each other,then the relative frequency response will be approximately zero over thefrequency band of interest, both in terms of relative magnitude andrelative phase.

Both the magnitude and the phase responses of the microphones arecritical to successful implementation of modern algorithms for speechenhancement and noise reduction. In some cases, it is not necessary toknow the individual phase response of a particular microphone; instead,the relative phase response between any two microphones is sufficientinformation for most of the algorithms to work properly. Accordingly, ifit were possible to determine the relative frequency response, includingthe relative magnitude and the relative phase responses, between twomicrophones, then such information can be used to compensate fordifferences between such microphones. Unlike magnitude responsemeasurement, relative phase response measurement between two microphonesis an extremely difficult problem. At higher frequencies, even a smallpositional variation in the measurement set-up (even of a fewmillimeters) can drastically affect the phase measurement results. Inorder to comply with ITU-T wide band mode standards, the relativefrequency response must be considered over at least the range of 100 Hzto 7000 Hz.

Various compensation techniques have been used in the past to accountfor microphones that differ in relative frequency response.Self-calibration is a technique used to adjusting the parameters of acompensation system using an excitation signal that is usually presentduring the normal mode of operation of the audio device. One example ofthis self-calibration technique is disclosed within Patent ApplicationPublication No. US 2004/0165735, published Aug. 26, 2004. On-linecalibration is a second technique adjusting the parameters of acompensation system, wherein the parameters of the calibrating systemare adaptively updated during the normal mode of operation. An exampleof this calibration technique is disclosed within U.S. Pat. No.6,914,989, issued to Janse, et al., on Jul. 5, 2005.

A third technique used to adjust the parameters of a compensation systemis known as “fixed calibration”; a fixed calibration technique refers tomeasuring the relative frequency response between a pair of microphonesusing an off-line process, and then initializing a fixed set ofcalibration parameters based on the measurement. One of the difficultiesof effectively implementing a fixed calibration compensation techniqueis accurately determining the relative frequency response as between twomicrophones. To accurately determine such relative frequency response asbetween two microphones within a frequency band of interest, one mustknow both the differences in the magnitude response of the twomicrophones as well as the differences in phase response of the twomicrophones.

In addition, within some audio devices, the audio path to the firstmicrophone and the audio path to the second microphone differ from eachother. Thus, even if the two microphones were themselves perfectlymatched to each other, the difference in the respective audio pathsleading to the first and second microphones may result in a relativefrequency response that needs to be compensated. In some audio products,the microphones are mounted deep inside the outer housing of theproduct. The frequency response of the installed microphones cansometimes drastically differ from the free standing frequency responseof each such microphone. Accordingly, the mechanical design of themicrophone housing, along with the acoustic path inside the product, cangreatly affect the overall frequency response of the acquired signalthat will be used for further signal processing.

Some of the factors that will affect the overall frequency response are:the acoustic tube length from the microphone hole in an audio product tothe port in the microphone capsule; multi-path acoustic leakage;resonant cavities; and improper microphone booting. Hence, for thoseproducts in which microphones are embedded deep within the outerhousing, it is important to measure the overall frequency response thatencompasses both the microphone itself and the acoustic path to themicrophone, rather than merely measuring the frequency response of thefree standing microphone. The measurement logistics are furthercomplicated by the fact that audio products using the same types ofmicrophones come in various shapes and sizes. The accessibility ofmicrophone holes further complicates the measurement process too.

Adding to the complexity of relative frequency response, there are avariety of different types of microphones in current use, includingelectret condenser microphones (or “ECMs”) and micro electro-mechanicalsystem (so-called “MEMS”) microphones. A cylindrically-shaped electret(ECM) microphone might have typical dimensions of 9.5 mm in diameter×6.3mm in height. In contrast, a cuboidal micro electro-mechanical system(MEMS) microphone would typically have much smaller dimensions, on theorder of 3.76 mm in length, ×3.0 mm in width, ×1.1 mm in height. A testapparatus used to detect the relative frequency response of microphoneswould need to be capable of accommodating at least both such types ofmicrophones.

One known technique for measuring the relative frequency response asbetween two microphones is to position both microphones within a testchamber, equidistant from a loudspeaker, and to alternately measure theresponse of each microphone to an excitation signal issued by theloudspeaker. However, the two microphones are positioned at twodifferent points in space within the test chamber, each having its ownunique propagation path. As already noted above, differences in thepropagation paths for two microphones can change the effective relativefrequency response of such microphones. It is therefore important tominimize any differences in the propagation paths to the microphonesunder test when designing a measurement set up in order to obtainaccurate results.

Theoretically, one could maintain the propagation path to the twomicrophones substantially constant by first positioning the firstmicrophone at a given point in the test chamber, measuring the frequencyresponse of the first microphone, then removing the first microphone,replacing it with the second microphone at the same given point, andmeasuring the frequency response of the second microphone. However, evensmall changes in a physical set up between two successive measurementscan alter the acoustic field. For example, the acoustic field can changewhen the first microphone is manually replaced by the second microphonein the measurement set up. A small change in the apparatus positionbetween the two measurements can also introduce different diffractionpatterns, thereby affecting the acoustic field at the sensing point.

Accordingly, it is an object of the present invention to provide a testset-up apparatus and method for measuring the phase and magnitudedifferences between first and second microphones for use in a fixedcalibration system in a manner that is non-destructive to themicrophones under test, and is reliable and repeatable, even when thetest set-up is disassembled and reassembled several times.

Another object of the present invention is to provide such a test set-upand method which provides a smooth response to measured magnitude andphase without extreme variations, especially at higher frequencies.

Still another object of the present invention is to provide such a testset-up and method capable of handling various microphone types andshapes, including both ECM-style and MEMS-style microphones.

A further object of the present invention is to provide such a testset-up and method capable of measuring the relative frequency responseof microphones over at least the frequency range of 100 Hz to 8,000 Hz.

A still further object of the present invention is to provide such atest set-up and method that is relatively simple to prepare and conduct,allowing such measurements to be completed in less than 30 minutes.

Yet a further object of the present invention is to provide such a testset-up and method which minimizes any changes in the acoustic field,while maintaining a consistent propagation path, when alternatingbetween measurements of frequency response of first and secondmicrophones.

Another object of the present invention is to provide such a test set-upand method capable of performing such relative frequency responsemeasurements even when two or more microphones are mounted deep insidean outer housing of an audio product.

These and other objects of the invention will become more apparent tothose skilled in the art as the description of the present inventionproceeds.

SUMMARY OF THE INVENTION

Briefly described, and in accordance with a preferred embodimentthereof, the present invention relates to an apparatus for measuring thephase and magnitude differences between first and second microphones inresponse to an excitation signal issued by an audible signal source. Amicrophone carrier is mounted for rotation about an axis of rotation.The rotatable microphone carrier has a mounting surface for supportingfirst and second microphones under test; preferably, the mountingsurface of the microphone carrier is smooth to minimize any diffractionof the excitation signal. The microphone carrier also has a smoothopposing surface opposite the mounting surface. The microphone carriercan be supported proximate to an audible signal source.

First and second apertures, symmetric about the axis of rotation, extendthrough the microphone carrier from the mounting surface to the smoothopposing surface; ideally, both the first and second symmetricalapertures are circular in cross-section. These first and secondsymmetrical apertures lie substantially co-linear with, and on oppositesides of, the axis of rotation of the microphone carrier, and areequidistant therefrom. A first portion of the mounting surfacesurrounding the first symmetrical aperture is adapted to sealinglyreceive the first microphone, while a second portion of the mountingsurface surrounding the second symmetrical aperture is adapted tosealingly receive the second microphone. Preferably, the firstmicrophone is sealingly mounted to the mounting surface to cover thefirst symmetrical aperture, while the second microphone is sealinglymounted to the mounting surface to cover the second fixed symmetricalaperture.

During testing, an audible signal source is preferably disposedproximate to the microphone carrier for issuing an excitation signalacross its smooth opposing surface. The microphone carrier can initiallybe rotated to a first position for placing the first microphone closestto the audible signal source, and the second microphone furthest fromthe audible signal source, in order to measure the excitation signalreceived by each of the first and second microphones. Thereafter, themicrophone carrier can be rotated 180 degrees to a second position forplacing the second microphone closest to the audible signal source, andthe first microphone furthest from the audible signal source, and theresponses of the respective microphones to the excitation signal aremeasured again.

It is preferred that the microphone carrier is of the form of agenerally circular disk having an outer perimeter, and that the firstand second apertures be located closer to the axis of rotation of themicrophone carrier than to its outer perimeter.

To aid in positioning and sealing the first microphone against themounting surface of the microphone carrier, in alignment with the firstsymmetrical aperture, a first gasket is provided. The first gasketincludes an opening for alignment with the first symmetrical aperture.The first gasket is placed against the mounting surface for forming aseal between the mounting surface and the first microphone, while theopening of the first gasket allows the excitation signal to pass throughthe first symmetrical aperture to the first microphone. Similarly, asecond gasket is also provided, having an opening for alignment with thesecond symmetrical aperture. The second gasket is placed against themounting surface for forming a seal between the mounting surface and thesecond microphone, while the opening of the second gasket allows theexcitation signal to pass through the second symmetrical aperture to thesecond microphone. Preferably, the openings formed in the first andsecond gaskets have the same general shape as the first and secondmicrophones for receiving the first and second microphones within suchopenings. The first and second gaskets may each be formed of anelongated sheet of resilient compressible material extending betweenfirst and second opposing ends. Preferably, the microphone-receivingopening formed in each such gasket is disposed closer to a first end ofeach elongated sheet than to the opposing second end, thereby allowing auser to manipulate the second end of each elongated sheet to align themicrophone received within its opening with one of the symmetricalapertures of the microphone carrier. To facilitate such alignment, themicrophone carrier is preferably formed of a translucent material forallowing a user to visualize the first and second microphones, and tovisualize the first and second gaskets, through the smooth opposingsurface of the microphone carrier.

In regard to another aspect of the present invention, a testingapparatus is provided for measuring the phase and magnitude differencesbetween at least first and second microphones each mounted within thehousing of an audio device. First and second apertures are formed withinthe outer housing of the audio device, corresponding to locations atwhich the first and second microphones are situated.

The testing apparatus includes a mounting plate having a mountingsurface and having a smooth opposing surface opposite the mountingsurface. The mounting plate is configured to be placed near an audiblesignal source. An aperture is provided within the mounting plate,extending from the mounting surface to the smooth opposing surface. Agasket is also provided, the gasket being placed against the mountingsurface for forming a seal between the mounting surface and the outerhousing of the audio device under test. The gasket includes an openingfor alignment with the aperture of the mounting plate.

The aforementioned testing apparatus also includes a clamp configured tophysically support the audio device at a desired angular position, andfurther includes a jack coupled to the clamp for selectively urging theclamp, and the device supported thereby, toward the gasket and themounting surface. The jack may be manipulated to initially urge theouter housing of the audio device against the gasket and mountingsurface for aligning the first aperture of the audio device with theaperture formed in the mounting plate and the opening in the gasket.Measurements can then be taken of the excitation signal received by thefirst microphone. Thereafter, the clamp and jack can be reconfigured tourge the outer housing of the audio device against the gasket andmounting surface, but now aligning the second aperture of the audiodevice with the aperture formed in the mounting plate and the opening inthe gasket. Measurements can then be taken of the excitation signalreceived by the second microphone. The results of such measurements canthen be used to calibrate the relative frequency response as between thefirst and second microphones.

As mentioned above, the present invention also relates to a method formeasuring the phase and magnitude differences between first and secondmicrophones in response to an excitation signal issued by an audiblesignal source. In practicing such method, a microphone carrier isprovided, wherein the microphone carrier has a mounting surface forsupporting first and second microphones and has a smooth opposingsurface opposite the mounting surface. In addition, first and secondsymmetrical apertures are provided within the microphone carrier spacedfrom each other, each extending through the microphone carrier from themounting surface to the smooth opposing surface. Preferably, the methodincludes the step of rotatably supporting the microphone carrier aboutan axis of rotation; in that instance, the first and second symmetricalapertures are preferably formed to be substantially co-linear with, andlying on opposite sides of, the axis of rotation of the microphonecarrier, and equidistant from its axis of rotation.

The aforementioned method also includes the steps of sealingly mountingthe first microphone against a portion of the mounting surfacesurrounding the first symmetrical aperture to cover the first fixedsymmetrical aperture, and sealingly mounting the second microphoneagainst a portion of the mounting surface surrounding the secondsymmetrical aperture to cover the second symmetrical aperture.Preferably, this step includes inserting a first gasket against themounting surface for forming a seal between the mounting surface and thefirst microphone, and aligning an opening of the first gasket with thefirst symmetrical aperture to allow the excitation signal to passthrough the first symmetrical aperture to the first microphone.Similarly, the step of inserting a second gasket against the mountingsurface for forming a seal between the mounting surface and the secondmicrophone may also be performed, wherein an opening of the secondgasket is aligned with the second symmetrical aperture to allow theexcitation signal to pass through the second symmetrical aperture to thesecond microphone.

It is preferred that the step of sealingly mounting the first microphoneagainst the mounting surface of the microphone carrier is performed byproviding an elongated sheet of resilient compressible materialextending between first and second opposing ends. A hole is formedwithin such sheet proximate to its first end for receiving the firstmicrophone. A foam block is then applied over the first microphone, andover the first end of the elongated sheet; the foam block is secured tothe microphone carrier to loosely retain the first microphone, and thefirst end of the elongated sheet, against the mounting surface of themicrophone carrier, while leaving the second end of the elongated sheetexposed. The second end of the elongated sheet is then manipulated toproperly align the first microphone relative to the first symmetricalaperture before firmly clamping the foam block against the mountingsurface of the microphone carrier.

The microphone carrier is initially oriented in a first orientation toposition the first symmetrical aperture relatively close to the audiblesignal source, and the excitation signal received by the firstmicrophone is measured. The microphone carrier is then re-oriented to asecond orientation to position the second symmetrical aperturerelatively close to the audible signal source, and the excitation signalreceived by the second microphone is measured. Based upon themeasurements made for the first and second microphones when themicrophone carrier is at its first and second orientations,respectively, the phase and magnitude differences between the first andsecond microphones are derived.

In the case wherein the microphone carrier is rotatably supported aboutan axis of rotation, the aforementioned step of re-orienting themicrophone carrier to position the second symmetrical aperturerelatively close to the audible signal source includes the step ofrotating the microphone carrier 180 angular degrees from its initialposition corresponding to the first symmetrical aperture beingrelatively close to the audible signal source.

In practicing the foregoing method, it may be advantageous to measurethe excitation signal received by each of the first and secondmicrophones when the microphone carrier is oriented at its firstorientation, and to measure the excitation signal received by each ofthe first and second microphones when the microphone carrier is orientedat its second orientation. The step of deriving phase and magnitudedifferences between the first and second microphones may then be basedupon the measurements acquired for both the first and second microphonesat both the first and second orientations of the microphone carrier.

As was also described above, another aspect of the present invention isto provide a method for measuring the phase and magnitude differencesbetween at least first and second microphones that are mounted inside anaudio device having an outer housing. In such instances, the outerhousing of the audio device typically has first and second aperturesformed therein, corresponding to locations at which first and secondmicrophones are situated. In practicing this method, a mounting plate isprovided having a mounting surface for engaging the outer housing of theaudio device; the mounting plate includes a smooth opposing surfaceopposite the mounting surface. An aperture is provided in the mountingplate, extending from the mounting surface to the smooth opposingsurface. An audible signal source is provided for issuing an excitationsignal proximate to the smooth opposing surface of the mounting plate.

The audio device under test is then positioned into a first positionwherein the first aperture of its outer housing is urged against themounting surface of the mounting plate in alignment with the aperture ofthe mounting plate. For example, a jack may be used to selectively pushthe audio device against the mounting surface. Preferably, as the firstaperture in the outer housing of the audio device is urged against themounting surface of the mounting plate, a seal is formed about the firstaperture of the outer housing of the audio device and the aperture ofthe mounting plate. The excitation signal received by the firstmicrophone of the device is then measured.

Thereafter, the audio device is re-positioned into a second position inwhich the second aperture of the outer housing of the audio device isurged against the mounting surface of the mounting plate in alignmentwith the aperture in the mounting plate. It is again preferred that, asthe second aperture in the outer housing of the audio device is urgedagainst the mounting surface of the mounting plate, a seal is formedabout the second aperture of the outer housing of the audio device andthe aperture of the mounting plate. The excitation signal received bythe second microphone of the device when the device is positioned in itssecond position is likewise measured. Phase and magnitude differences asbetween the first and second microphones are then derived based upon themeasurements acquired with the audio device at its first and secondpositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a microphone carrier plate used tosupport a pair of microphones under test.

FIG. 2 is a cross-sectional view of the microphone carrier plate withfirst and second microphones secured thereto.

FIG. 3 is a top view of the microphone carrier plate shown in FIGS. 1and 2.

FIG. 4 is a side view of a test chamber in which the microphone carrierplate of FIG. 1 has been placed for taking measurements.

FIG. 5 is a perspective view of a pair of gasket strips used to receiveand align a pair of microphones.

FIGS. 6A and 6B are schematic drawings showing two alternate testpositions for taking frequency response measurements of first and secondmicrophones, one test position being rotated 180 degrees relative to theother.

FIG. 7 is a perspective view of an audio device having an outer housingwith first and second apertures for guiding sound to first and secondmicrophones disposed within such outer housing.

FIG. 8 is a perspective view of a test apparatus including a clampsupported upon an elevation jack for urging an audio device toward anupper mounting plate.

FIG. 9 is a side view of the apparatus shown in FIG. 8 wherein the audiodevice of FIG. 7 is held within the clamp for urging one of its firstand second apertures into sealing engagement with an aperture of the topplate.

FIG. 10 is an enlarged cross-sectional view of the upper portion of theaudio device shown in FIG. 9

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred form of apparatus for measuring the phase and magnitudedifferences between first and second microphones in response to anaudible excitation signal includes a microphone carrier plate asdesignated generally in FIG. 1 by reference numeral 20. Carrier plate 20is a circular plate or disk, and is preferably formed of transparent ortranslucent material, for reasons which will be explained below. A diskshape is preferred to maintain uniform and symmetric diffractions ofsound signals around the edges of carrier plate 20 in order to captureconsistent data at different rotation angles. Preferably, carrier plate20 has a radius measuring approximately 80 mm, and is made of eitherclear acrylic or Lexan-brand plexiglass. Carrier plate 20 includes acentral hole 22 for receiving the shaft of retaining screw 24. Carrierplate 20 includes a generally planar lower mounting surface 26 forsupporting first and second microphones (not shown in FIG. 1), and asmooth opposing upper surface 28 opposite lower mounting surface 26.Preferably, central hole 22 is beveled to fully receive the head ofretaining screw 24, whereby the head of retaining screw 24 liesessentially in the same plane as upper surface 28. Flush mounting of thehead of retaining screw 24 helps to avoid interference with the acousticfield along the upper surface 28 of carrier plate 20. Carrier plate 20is attached firmly to retaining screw 24 using a disk locknut 46 toprevent carrier plate 20 from spinning around retaining screw 24.Preferably, disk locknut 46 has a round shape to provide clearance formicrophone holders to be described below.

First aperture 27 is formed in carrier plate 20 a predetermined distancefrom central hole 22 and extends through carrier plate 20 from uppersurface 28 down to mounting surface 26. First aperture 27 permits anaudible excitation signal to pass therethrough to a first microphone(not shown in FIG. 1). Likewise, a second aperture 30 is formed incarrier plate 20 the same predetermined distance from central hole 22,but on the opposite side of central hole 22 as first aperture 27, andextends through carrier plate 20 from upper surface 28 down to mountingsurface 26. Both apertures 27 and 30 are essentially cylindrical,circular in cross-section, and symmetrical. First aperture 27 and secondaperture 30 are substantially co-linear with, and lie on opposite sidesof, central hole 22. As will be described below, central hole 20corresponds to an axis of rotation 32 of microphone carrier plate 20,and first aperture 27 and second aperture 30 are equidistant from axisof rotation 32. It is preferred that apertures 27 and 30 each lie closerto the central axis of rotation 32 than to the outer perimeter ofcarrier plate 20.

First aperture 27 permits an audible excitation signal transmitted nearupper surface 28 of carrier plate 20 to pass downwardly therethrough toa first microphone (not shown in FIG. 1). Similarly, second aperture 30permits the audible excitation signal to pass downwardly therethrough toa second microphone (not shown in FIG. 1). Preferably, first and secondapertures 27 and 30 are spaced approximately 15 mm from each other, andeach is spaced approximately 7.5 mm from axis of rotation 32. In thepreferred embodiment, the inner diameter of first and second apertures27 and 30 is approximately 3 mm.

Still referring to FIG. 1, a foam block 34 has a central hole 36 formedtherethrough for receiving the shaft of retaining screw 24. Foam block34 includes an upper surface 38 which is ultimately clamped againstmounting surface 26 of carrier plate 20 to secure first and secondmicrophones to carrier plate 20. Foam block 34 is preferably formed of acollapsible piece of foam and is also referred to herein as a conformalretainer. Foam block 34 is placed underneath the microphones under test.A closed-cell type foam is preferred over an open-cell type foam toavoid leakage of sound waves from the bottom of carrier plate 20. Foamblock 34 serves as an acoustic barrier to prevent sound signals fromreaching the microphones under test other than through apertures 27 and30.

Also shown in FIG. 1 is a bottom clamp 40, preferably formed ofaluminum. Clamp 40 is generally cylindrical, but has a rectangularchannel 42 formed therein for receiving the lower portion of foam block34. Bottom clamp 40 has a central aperture 44 for allowing the lower endof the shaft of retaining screw 24 to extend therethrough. The purposeof foam block 34 is to hold first and second microphones in positionagainst mounting surface 26 of carrier plate 20 by transferring forcefrom bottom clamp 40 in a non-destructive manner. Foam block 34 willmodify its shape under pressure to conform to different shapes and sizesof particular microphones under test. The flexibility of suchcollapsible foam allows for testing of microphones of different heightsand styles.

Turning to FIG. 2, the carrier plate 20, foam block 34, and bottom clamp40 are shown in a cross-sectional side view, along with first microphone50 and second microphone 52. First microphone 50 is positioned againstmounting surface 26 of carrier plate 20, directly below first aperture27 and covering such aperture. Second microphone 52 is also positionedagainst mounting surface 26 of carrier plate 20, directly below secondaperture 30 and covering such aperture. The first and second microphonesunder test, i.e., microphones 50 and 52, are placed in microphoneholders 56 and 58. Examples of such microphone holders are shown in FIG.5, wherein microphone holder 53 has a square-shaped hole 55 formed nearone end thereof for receiving a square-shaped microphone, and microphoneholder 57 has a circular hole 59 formed near one of its ends forreceiving a circular-shaped microphone. Each such microphone holder ispreferably custom-made for each microphone's unique size and shape.Preferably, such microphone holders are made from sheets of resilientcompressible gasket material. During use, the holes formed in microphoneholders 56 and 58 are aligned with apertures 27 and 30 for allowingsound passing through such apertures to reach microphones 50 and 52. Theopenings formed in microphone holders for receiving microphones 50 and52 are disposed closer to one end of the elongated sheet than the other.This allows a user to manipulate the second end of each elongated sheetto align the microphone received therein with the respective aperture(27/30) of carrier plate 20. Also, as shown in FIGS. 3 and 5, indexingmarks 61A/63A and 61B/63B may be formed on microphone holders 57/53,respectively, extending at right angles to each other, and intersectingat the center of holes 59/55, respectively, to aid in alignment of holes59/55 with the corresponding apertures 27/30 in plate 20. In addition,as shown in FIGS. 3 and 5, each of such microphone holders has acircular cut-out 65A/65B at its end closest to its microphone hole forallowing such microphone holders to abut the outer perimeter of disklocknut 46, if desired.

Referring again to FIG. 2, and also FIG. 3, microphone holder 56 formsan acoustic seal around microphone 50 relative to mounting surface 26 ofcarrier plate 20. Similarly, microphone holder 58 forms an acoustic sealaround microphone 52 relative to mounting surface 26 of carrier plate20. In addition, microphone holders 56 and 58 provide a simple andconvenient mechanism to properly position and align microphones 50 and52 with the respective first and second apertures 27 and 30 in carrierplate 20. Before tightening bottom clamp 40, one may manipulate thedistal ends of microphone holders 56 and 58, which project outwardlyfrom foam block 34, until each microphone (50/52) is centered below itsrespective aperture (27/30).

It was earlier mentioned that carrier plate 20 is preferably formed of amaterial that is transparent or translucent. This is because a user,looking downwardly at upper surface 28 of carrier plate 20 (see FIG. 3)may then simultaneously view microphones 50/52, microphone holders56/58, and apertures 27 and 30, to facilitate precise alignment ofmicrophones 50 and 52 with apertures 27 and 30, respectively. Thus,microphone holders 56 and 58 serve as both acoustic gaskets and aspositioning mechanisms for aligning microphones 50 and 52 with apertures27 and 30. Moreover, as indicated in FIG. 5, the use of two microphoneholders 56 and 58 allows for concurrent testing of two microphoneshaving two different shapes from each other. The use of such microphoneholders avoids the need for installation of custom made “boots”, andalso facilitates flush mounting of microphones 50 and 52 below carrierplate 20, against mounting surface 26, in essentially the same manner aseach other; this helps insure that the propagation paths to eachmicrophone are as identical as possible.

Within FIG. 3, line 64 is printed upon the upper surface of carrierplate 20 and indicates a diametrical line extending through the centerof carrier plate 20, and passing through both the centers of apertures27 and 30. Line 66 is also preferably printed upon the upper surface ofcarrier plate 20 and indicates a perpendicular axis passing through thecenter of carrier plate 20 and forming a right angle with diametricalline 64. Lines 64 and 66 assist a user in accurately determining theangular orientation of carrier plate 20, and for aligning themicrophones in the sound field during testing. Also printed upon theupper surface of carrier plate 20 is a circular ring 67 that is centeredupon the axis of rotation of carrier plate 20 (corresponding in FIG. 3with the intersection of alignment lines 64 and 66) and having adiameter equal to the spacing between apertures 27 and 30. When aligningmicrophone holders 56 and 58 with carrier plate 20 (thereby aligningmicrophones 50 and 52 with apertures 27 and 30), a user may easilymatch-up indexing lines 61A/63A and 61B/63B (see FIG. 5) with referenceline 64 and ring 67 printed on carrier plate 20 to achieve properalignment of microphones 50 and 52 with corresponding apertures 27 and30.

After a user confirms that microphones 50 and 52 are properly alignedwith apertures 27 and 30, bottom clamp 40 is tightened to apply upwardforce on foam block 34. Foam block 34 is compressed against microphones50/52 and the inner portions of microphone holders 56 and 58 toeffectively seal off such microphones, with the exception of any soundsignals entering apertures 27 and 30. Compressive force is applied bybottom clamp 40 to foam block 34 by retaining screw 24, and by the headof clamp screw 60. Clamp screw 60 is somewhat larger in diameter thanthe shaft of retaining screw 24. As shown in FIG. 2, the head 64 ofclamp screw 60 includes an internally-threaded bore 62 for receiving thelower end of retaining screw 24. As retaining screw 24 is tightenedwithin bore 62 of head 64, head 64 bears against bottom clamp 40,forcing it toward carrier plate 20. The pressure thereby applied by foamblock 34 ensures that sound waves reach the diaphragms of microphones 50and 52 only through apertures 27 and 30, respectively, and not from thesides. As shown in FIG. 2, clamp screw 60 extends along rotational axis32.

Now referring to FIG. 4, the assembly of FIG. 3 is placed inside ananechoic chamber 68 to generate a consistent acoustic field with verylittle ambient acoustic noise. Chamber 68 may take the form of a boxedenclosure with a door formed by one of the four sidewalls (e.g., thefront-most wall relative to FIG. 4) for inserting and removing carrierplate 20. As shown in FIG. 4, anechoic chamber 68 includes a floor 70,opposing end walls 72 and 74, back wall 76, and top 78. Loudspeaker 80is mounted within chamber 68 to play audio excitation signals;loudspeaker 80 is preferably spaced approximately 360 mm from theclosest edge of carrier plate 20 and sends the excitation signal acrossthe smooth upper surface 28 of carrier plate 20. The sidewalls, floorand top of chamber 68 are preferably acoustically treated bysound-absorbing layer 82 (e.g., anechoic wedges).

Still referring to FIG. 4, a programmable stepper motor 84 is supportedupon a rigid base platform 86. Elongated cylindrical shaft 88 extendsupwardly from stepper motor 84. The angle of rotation of shaft 88 iscontrolled by stepper motor 84, and can be programmed via a connectionto a personal computer (not shown). Shaft 88 is partially supported by astabilizer bearing assembly 90, in turn, supported by bracket 92.Stabilizer bearing assembly aids in preventing shaft 88 from bending ortilting out of a vertical axial alignment. The uppermost end of shaft 88includes an internally-threaded bore 94 for threadedly engaging thelower end of clamp screw 60.

When preparing to make measurements, the microphones 50 and 52 undertest are first positioned below carrier plate in the manner describedabove. After properly aligning microphones 50 and 52 with apertures 27and 30, retaining screw 24 is tightened into the head of clamp screw 60to firmly secure foam block 34 against the undersides of themicrophones, against microphone holders 56 and 58, and against mountingsurface 26, to eliminate acoustic leakage paths. Carrier plate 20 isthen mounted upon stepper motor rotation shaft 88 by threadedly engagingclamp screw 60 therewith, thereby allowing carrier plate 20 to berotated about rotation axis 32.

As shown in FIG. 6A, the starting position of stepper motor shaft 88 isadjusted such that microphone axis 64 (see FIG. 3) is aligned to thecenter axis 96 of loudspeaker 80. Microphone 50 is initially positionedcloser to loudspeaker 80, and microphone 52 is initially positionedfurthest from loudspeaker 80. A spectrally flat (so-called “white”)excitation signal is then played through loudspeaker 80. Two microphonesignals, designated x₁₁(t) and x₂₁(t) are acquired in this position, onefor microphone 50 and the other for microphone 52, representing thefrequency response of each microphone. While not shown in FIG. 4, a pairof wires extends from each microphone under test for taking suchmeasurements; those wires are guided through small openings in chamber68 for allowing such measurements to be recorded.

Thereafter, stepper motor 84 is operated to rotate shaft 88, and carrierplate 20, exactly 180 degrees, so that as shown in FIG. 6B, microphone52 now lies closest to loudspeaker 80, and microphone 50 now liesfurthest from loudspeaker 80. The excitation signal is again played byloudspeaker 80, and the responses of the two microphones are againrecorded. Carrier plate 20 is maintained in a horizontal planethroughout its rotation, always lying perpendicular to rotator shaft 88.Also, apertures 27 and 30 are precisely drilled to ensure that they arealigned with the center of carrier plate 20 and equidistant therefrom.Accordingly, in regard to FIG. 6B, microphone 52 now lies in preciselythe same spatial position as was true for microphone 50 in the initialtest shown in FIG. 6A. Similarly, in regard to FIG. 6B, microphone 50now lies in precisely the same spatial position as was true formicrophone 52 in FIG. 6A. Use of the stepper motor to swap positions ofthe microphones allows for measurement of the sound pressure at the samespatial point by both microphones without perturbing the acoustic field.Two more microphone signals X₁₂(t) and X₂₂(t) are acquired in therotated position shown in FIG. 6B.

If a signal x(t) is played through a loudspeaker, the frequency responseof the received signal y(t) acquired using a microphone amplified by apreamplifier can be written as:

Y(f)=X(f)H _(L)(f)H _(pp)(f)H _(m)(f)H _(pa)(f)

wherein H_(L)(f) is the frequency response of the loudspeaker; H_(pp)(f)models the acoustic propagation path from the loudspeaker to themicrophone; H_(m)(f) is the frequency response of the microphone; andH_(pa)(f) is the frequency response of the preamplifier. If twomicrophones are tested for frequency response, using the sameloudspeaker, the same propagation path, and the same preamplifier, thenthe relative frequency response, or H_(d)(f) of the two microphones canbe expressed as a ratio wherein the frequency responses of theloudspeaker, propagation path, and preamplifier cancel each other out,leaving:

H _(d)(f)=H _(m1)(f)/H _(m2)(f)

wherein H_(m1)(f) represents the frequency response of the firstmicrophone, and H_(m2)(f) represents the frequency response of thesecond microphone. The above equation can be rewritten to separately setforth the magnitude and phase responses of the two microphones asfollows:

${H_{d}(f)} = {{\frac{H_{m\; 1}(f)}{H_{m\; 2}(f)}}e^{j{({{\Phi_{m\; 1}{(f)}} - {\Phi_{m\; 2}{(f)}}})}}}$

wherein H_(m1)(f) and H_(m2)(f) represent the magnitude of the responsesof the first and second microphones as a function of frequency, andΦ_(m1) and Φ_(m2) represent the phase responses of the first and secondmicrophones as a function of frequency.

With reference to FIGS. 6A and 6B, the relative frequency responseestimate for microphones 50 and 52 can be obtained by averaging themeasurements obtained from both of the positions (i.e., the firstposition shown in FIG. 6A, and the second position shown in FIG. 6B).Specifically,

${{H_{d\; 1}(f)} = {{\frac{X_{1\; 1}(f)}{X_{22}(f)}}e^{j{\{{{\Phi_{11}{(f)}} - {\Phi_{22}{(f)}}}\}}}}},{{H_{d\; 2}(f)} = {{\frac{X_{12}(f)}{X_{21}(f)}}e^{j{\{{{\Phi_{12}{(f)}} - {\Phi_{21}{(f)}}}\}}}}},{{{\hat{H}}_{d}(f)} = {{\frac{1}{2}\lbrack {{H_{d\; 1}(f)} + {H_{d\; 2}(f)}} \rbrack} = {{{{\hat{H}}_{d}(f)}}e^{j\; {{\hat{\Phi}}_{d}{(f)}}}}}}$

where X_(ij)(f) is the power spectral density of microphone i at j^(th)measurement and Ĥ_(d)(f) is the relative frequency response estimatebetween the two microphones under test. In this manner, phase andmagnitude differences between the first and second microphones may bederived, and appropriate compensation schemes may be implemented withinthe audio device that will be using such microphones.

It should be understood that, while the test apparatus shown in FIG. 4may induce certain diffraction effects, such diffraction effects willnot affect the final result because it is the relative frequencyresponse between the two microphones that is being measured. This istrue provided that the acoustic field remains the same between these twomeasurements. As described above, small changes in a physical set upbetween two successive measurements can alter the acoustic field. Forexample, the acoustic field can change when the first microphone ismanually replaced by the second microphone in the measurement set up. Asmall change in the apparatus position between the two measurements canalso introduce different diffraction patterns, thereby affecting theacoustic field at the sensing point. However, the rotating carrierplate/stepper motor technique described above reduces measurement errorvariations by placing both microphones in the test chamber beforetesting begins, taking measurements from both microphones withoutdisturbing the physical set up, and by successively placing bothmicrophones at the same spatial point to sense the sound pressure at thevery same point.

The test set-up and related method described above with regard to FIGS.1-6 works well for two discrete microphones. However, as mentionedabove, there are instances wherein a pair of microphones is mountedrelatively deeply within the housing of an audio product, in which casethe relative frequency response of the two microphones will besignificantly impacted by their respective propagation paths, even ifthe two microphones are perfectly matched to each other. For example,FIG. 7 illustrates the upper portion of an audio product 100 having afirst microphone 150 and a second microphone 152. The outer housing ofaudio product 100 includes a first port 127 for sending sounds, along afirst propagation path 131, to internal microphone 150 mounted furtherinside audio product 100. Similarly, second port 130 sends sounds, alonga second propagation path 133, to internal microphone 152, also mountedfurther inside audio product 100. In this case, the only reliable way todetermine the relative frequency response of microphones 150 and 152 isto measure the responses of such microphones as installed in audioproduct 100. In the case of some audio products, the first and secondmicrophone holes may even be located on different surfaces of thehousing, and may be pointed in different directions.

Referring to FIGS. 8 and 9, a test apparatus is shown for use indetermining the relative frequency response of two internal microphoneswhile minimizing the influence of the case, or housing, of the audioproduct 300 when measuring the microphone response. The testingapparatus includes a transparent boxed enclosure, or test chamber, 168,having a door (not shown) formed by one of its sidewalls. The dimensionsof chamber 168 are preferably 550 mm in length×350 mm in width×320 mm inheight. The three fixed side walls are preferably constructed with 10 mmthick double layered plexiglass. The double layered walls are used toreduce the effects of acoustic vibrations that might otherwise leak intochamber 168. As in the case of chamber 68 shown in FIG. 4, thesidewalls, door, and floor, of chamber 168 are preferably covered withsound absorbing acoustical treatment to suppress extraneous sound waves.Sound is permitted to enter into test chamber 168 only through a hole182 formed through the top plate 178 Test chamber 168 is constructed inan acoustically sealed manner such that sound originating outside thechamber may only enter the interior of the chamber through top platehole 182.

Positioned upon floor 170 of test chamber 168 is a lab jack 200. Labjack 200 is a scissors-action type jack having a rigid base 201.Rotation of height adjustment knob 202 raises or lowers jack platform204, as indicated by arrows 206. An XY positioner 208 includes a base210 secured upon jack platform 204. Extending upwardly from base 210 isa stub shaft 212, the lower end of which forms a locking ball joint withbase 210. Locking knob 214 may be loosened temporarily to move stubshaft 212 to a desired tilt angle, if needed, and then re-tightened. Theupper end of stub shaft 212 is secured to a fixed block 216. Movableblocks 218 and 220 are coupled to fixed block 216 by a pair of sliderods 222 and 224, together with a threaded drive rod 226. Crank handle228 may be rotated to move blocks 218 and 220 toward, or away from,fixed block 216. L-shaped clamping members 230 and 232 are securedrespectively to the tops of movable blocks 218 and 220. Thus, a user canrotate crank 228 to clamp a device under test (300) between clampingmembers 230 and 232. The height of the device under test can be adjustedvia adjustment knob 202 of lab jack 200, and the tilt angle of the audiodevice under test can be adjusted by unlocking, and manipulating theball joint formed between stub shaft 212 and base 210, allowing theaudio device to tilt to any desired angle in both x and y directions.

As shown in FIGS. 9 and 10, test chamber 168 includes top plate 178having a top plate hole 182 formed therein. Top plate 178 is preferablymade of 5 mm thick plexiglass or conventional glass. Top plate hole 182preferably has a 3 mm diameter. Within FIGS. 9 and 10, the audio deviceunder test is designated by reference numeral 300, and in FIG. 10,reference numeral 327 designates a port, or microphone hole, in thehousing of audio device 300 leading to an internal microphone beingmeasured. It is preferred that top plate 178 be transparent, ortranslucent, for allowing a user to observe the microphone hole 327 ofthe audio product 300, relative to top plate hole 182, to achieveprecise alignment therebetween. The lower surface of top plate 178 maybe regarded as a mounting surface; the upper surface of top plate 178 ispreferably smooth and planar.

To obtain reliable measurements, microphone hole 327 (see FIG. 10) ofaudio product 300 must be placed under top plate hole 182 with a verygood seal around such hole. If sound waves were to travel inside testchamber 168 to microphone hole 327, other than through top plate hole182, large variations in the acoustic field would result which couldgreatly affect the measured frequency response of a microphone. In orderto reduce such leakage paths, a gasket 356, preferably made from a clearsilicone, adhesive-backed sheet, is attached directly underneath topplate hole 182. A hole 357 is punched through silicone sheet 356 inalignment with, and of the same diameter as, top plate hole 182. Byapplying sufficient upward force to audio device 300, as by raisingplatform 204 of lab jack 200, silicone gasket 356 compresses to providethe necessary acoustic seal between the housing of audio device 300 andtop plate 178 adjacent top plate hole 182.

When measurements are to be made of the frequency response ofmicrophones within audio device 300, a loudspeaker 180 is mounted nearthe top of test chamber 168 to play excitation signals across top plate178. Loudspeaker 180 is physically separated from test chamber 168 tominimize any mechanical vibrations that might otherwise be coupled tothe microphone under test. Audio device 300 is clamped within the jawsof clamp 208 and the position of audio device 300 is adjusted to placethe appropriate microphone hole (e.g., microphone hole 327 in FIG. 10)under, and in alignment with, top plate hole 182. The height of lab jack200 is adjusted to obtain a proper seal between gasket 356 andmicrophone hole 327. The door to test chamber 168 is then closed, and aspectrally flat excitation signal is played by loudspeaker 180 acrosstop plate 178.

While not shown in the drawing figures, electrical wires extend fromaudio device 300, and outwardly through acoustically sealed bulkheads inone of the walls of test chamber 168, for allowing the response of theinternal microphone to be measured. After measurements are obtained forthe first microphone hole, the above-described procedure is repeated,this time positioning a second microphone hole under, and in alignmentwith, top plate hole 182. Loudspeaker 180 then plays the same excitationsignal as before across top plate 178, and measurements are obtained forthe second microphone. It is important to note that loudspeaker 180 mustremain in the same position during testing of the first microphone andtesting of the second microphone. This will insure that the soundpressure produced by loudspeaker 180 at top plate hole 182 is the samesound pressure during measurement of the first microphone as duringmeasurement of the second microphone. The relative frequency responsecan then be estimated using the method explained above.

The location of the microphone hole in a device can pose manychallenges. Sometimes, a proper seal can only be formed between amicrophone hole and top plate hole 182 when audio device 300 is standingat a certain slanting angle, as shown for example in FIG. 10. At thisslanting angle, the upward force applied from the lab jack to the audiodevice may cause the audio device to slip away from top plate hole 182or otherwise compromise the desired seal around top plate hole 182. Tomitigate this problem, a retaining member 190 may be secured along topplate 178. Retaining member 190 has a length selected to match the audiodevice under test, whereby edge 192 of retaining member 190 abuts thehousing of audio device 300 to rest against the device under test andprevent it from slipping laterally during the measurement process. Theapparatus described above with regard to FIGS. 8-10 appears to besuitable for testing a wide variety of audio products having differentshapes, sizes, and microphone hole configurations.

The apparatus and related method described above relative to FIGS. 7-10of the drawings has assumed that the microphones under test are embeddedwithin a housing of an audio product. Those skilled in the art shouldappreciate, however, that the same apparatus and method may be used todetermine the relative frequency response of two stand-alonemicrophones, if desired. The first stand-alone microphone can besupported within a test housing (similar to housing 100 shown in FIG. 7)just behind housing aperture 127; the test housing may then be clampedagainst plate 178 and gasket 356 (see FIGS. 9 and 10), while centeringtest housing aperture 127 with top plate hole 182 in plate 178 and withaperture 357 of gasket 356. Frequency response measurements of the firstmicrophone are then made. The first microphone is then removed from thetest housing, and a second stand-alone microphone is then supportedwithin the same test housing, once again just behind housing aperture127. The test housing is again clamped against top plate 178 and gasket356 (see FIGS. 9 and 10), while centering test housing aperture 127 withaperture 182 in plate 178 and with aperture 357 of gasket 356. Frequencyresponse measurements are now taken for the second microphone. Thisprocedure may be repeated for as many separate microphones are used fora given design.

Those skilled in the art will now appreciate that a simple but effectiveapparatus and method have been described for measuring the phase andmagnitude differences between first and second microphones for use in afixed calibration system of an audio product. The disclosed measurementset-up and related method are non-destructive to the microphones undertest. Such measurements can be made reliably and repeatably even afterdisassembling, and reassembling, the test set-up. The test set-up andmethods described above accommodate a wide variety of microphone typesand shapes, and allows relative frequency response measurements to becompleted easily and quickly. In the case of testing discretemicrophones, changes in the acoustic field are avoided, and thepropagation path is maintained consistent. Apparatus and methods forreliably measuring relative frequency response between two or moremicrophones that are mounted deep inside an outer housing of an audioproduct have also been disclosed above.

While the present invention has been described with respect to preferredembodiments thereof, such description is for illustrative purposes only,and is not to be construed as limiting the scope of the invention.Various modifications and changes may be made to the describedembodiments by those skilled in the art without departing from the truespirit and scope of the invention as defined by the appended claims.

We claim:
 1. An apparatus for measuring the phase and magnitudedifferences between first and second microphones in response to anexcitation signal issued by an audible signal source, comprising: a) amicrophone carrier mounted for rotation about an axis of rotation, therotatable microphone carrier having a mounting surface for supportingfirst and second microphones, and the microphone carrier having a smoothopposing surface opposite the mounting surface, the microphone carrierbeing configured to be placed proximate to an audible signal source; b)a first symmetrical aperture extending through the microphone carrierfrom the mounting surface to the smooth opposing surface, a portion ofthe mounting surface surrounding the first symmetrical aperture beingadapted to sealingly receive the first microphone; c) a secondsymmetrical aperture extending through the microphone carrier from themounting surface to the smooth opposing surface, a portion of themounting surface surrounding the second symmetrical aperture beingadapted to sealingly receive the second microphone; and d) the first andsecond symmetrical apertures being substantially co-linear with, andlying on opposite sides of, the axis of rotation of the microphonecarrier, and being equidistant from the axis of rotation; whereby themicrophone carrier may be rotated to a first position for placing thefirst microphone closest to the audible signal source in order tomeasure the excitation signal received by each of the first and secondmicrophones when the first microphone is closest to the audible signalsource, and whereby the microphone carrier may also be rotated to asecond position for placing the second microphone closest to the audiblesignal source in order to measure the excitation signal received by eachof the first and second microphones when the second microphone isclosest to the audible signal source.
 2. The apparatus recited by claim1 further including: a) a first microphone sealingly mounted to themounting surface to cover the first symmetrical aperture; and b) asecond microphone sealingly mounted to the mounting surface to cover thesecond fixed symmetrical aperture.
 3. The apparatus recited by claim 1further including an audible signal source for issuing an excitationsignal across the smooth opposing surface of the microphone carrier. 4.The apparatus recited by claim 1 wherein the mounting surface of themicrophone carrier is smooth to minimize diffraction of the excitationsignal.
 5. The apparatus recited by claim 1 wherein the first and secondsymmetrical apertures are both circular in cross-section.
 6. Theapparatus recited by claim 1 wherein the first symmetrical aperture isformed at a first fixed location within the microphone carrier, andwherein the second symmetrical aperture is formed at a second fixedlocation within the microphone carrier.
 7. The apparatus recited byclaim 1 wherein the microphone carrier has an outer perimeter relativeto its axis of rotation, and wherein the first symmetrical aperture andthe second symmetrical aperture lie closer to the axis of rotation thanto the outer perimeter.
 8. The apparatus recited by claim 1 furtherincluding: a) a first gasket having an opening, the first gasket beingplaced against the mounting surface for forming a seal between themounting surface and the first microphone, the opening of the firstgasket being aligned with the first symmetrical aperture for allowingthe excitation signal to pass through the first symmetrical aperture tothe first microphone; and b) a second gasket having an opening, thesecond gasket being placed against the mounting surface for forming aseal between the mounting surface and the second microphone, the openingof the second gasket being aligned with the second symmetrical aperturefor allowing the excitation signal to pass through the secondsymmetrical aperture to the second microphone.
 9. The apparatus recitedby claim 8 wherein the first and second microphones each have apredetermined shape, and wherein the first and second openings withinthe first and second gaskets have the same shapes as the first andsecond microphones, respectively, the opening formed in the first gasketreceiving the first microphone, and the opening in the second gasketreceiving the second microphone, each first and second gasket beingformed of an elongated sheet of resilient compressible materialextending between first and second opposing ends, the opening formed ineach such gasket being disposed proximate to the first end of theelongated sheet, allowing a user to manipulate the second end of eachelongated sheet to align a microphone receiving within the openingthereof with a symmetrical aperture of the microphone carrier.
 10. Theapparatus recited by claim 9 wherein the microphone carrier is formed ofa translucent material for allowing a user to visualize the first andsecond microphones, and to visualize the first and second gaskets,through the smooth opposing surface of the microphone carrier.
 11. Amethod for measuring the phase and magnitude differences between firstand second microphones in response to an excitation signal issued by anaudible signal source, comprising the steps of: a) providing amicrophone carrier, the microphone carrier having a mounting surface forsupporting first and second microphones and having a smooth opposingsurface opposite the mounting surface; b) rotatably supporting themicrophone carrier about an axis of rotation; c) providing first andsecond symmetrical apertures each extending through the microphonecarrier from the mounting surface to the smooth opposing surface, thefirst and second symmetrical apertures being substantially co-linearwith, and lying on opposite sides of, the axis of rotation of themicrophone carrier, and equidistant from the axis of rotation; d)sealingly mounting the first microphone against a portion of themounting surface surrounding the first symmetrical aperture to cover thefirst fixed symmetrical aperture; e) sealingly mounting the secondmicrophone against a portion of the mounting surface surrounding thesecond symmetrical aperture to cover the second symmetrical aperture; f)orienting the microphone carrier to position the first symmetricalaperture relatively close to the audible signal source, and measuringthe excitation signal received by the first microphone; g) re-orientingthe microphone carrier to position the second symmetrical aperturerelatively close to the audible signal source, and measuring theexcitation signal received by the second microphone; and h) derivingphase and magnitude differences between the first and second microphonesfrom the signals measured in steps f) and g).
 12. The method recited byclaim 11 wherein the step of re-orienting the microphone carrier toposition the second symmetrical aperture relatively close to the audiblesignal source includes the step of rotating the microphone carrier 180angular degrees from its position corresponding to the first symmetricalaperture being relatively close to the audible signal source.
 13. Themethod recited by claim 11 wherein: a) the step of orienting themicrophone carrier to position the first symmetrical aperture relativelyclose to the audible signal source includes the steps of measuring theexcitation signal received by each of the first and second microphones;and b) the step of re-orienting the microphone carrier to position thesecond symmetrical aperture relatively close to the audible signalsource includes the steps of measuring the excitation signal received byeach of the first and second microphones; and c) the step of derivingphase and magnitude differences between the first and second microphonesis based upon the measurements set forth in steps a) and b) herein. 14.The method recited by claim 11 wherein: the step of sealingly mountingthe first microphone against a portion of the mounting surfacesurrounding the first symmetrical aperture includes the step ofinserting a first gasket against the mounting surface for forming a sealbetween the mounting surface and the first microphone, and aligning anopening of the first gasket with the first symmetrical aperture forallowing the excitation signal to pass through the first symmetricalaperture to the first microphone; and the step of sealingly mounting thesecond microphone against a portion of the mounting surface surroundingthe second symmetrical aperture includes the step of inserting a secondgasket against the mounting surface for forming a seal between themounting surface and the second microphone, and aligning an opening ofthe second gasket with the second symmetrical aperture for allowing theexcitation signal to pass through the second symmetrical aperture to thesecond microphone.
 15. The method recited by claim 14 wherein the stepof sealingly mounting the first microphone against the mounting surfaceincludes the steps of: a) providing an elongated sheet of resilientcompressible material extending between first and second opposing ends;b) forming a hole proximate to the first end of the elongated sheet forreceiving a microphone; c) applying a foam block against the firstmicrophone and against the first end of the elongated sheet to urge thefirst microphone and the first end of the elongated sheet against themounting surface of the microphone carrier, while leaving the second endof the elongated sheet exposed; d) manipulating the second end of theelongated sheet to properly align the first microphone relative to thefirst symmetrical aperture; and e) thereafter clamping the foam blockagainst the mounting surface of the microphone carrier.